1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images and to a two-component developer, an image forming method and an image forming apparatus that use the same.
2. Description of the Related Art
In an electrophotographic apparatus or electrostatic recording apparatus, a toner is attached to a latent electrostatic image formed on a photoconductor, the toner-attached image is transferred onto a transfer material, then the toner is thermally fixed onto the transfer material to yield a toner image. In full-color image formation, in general, colors are reproduced using toners of four colors, i.e. black, yellow, magenta and cyan, and a full-color image is obtained by developing each color, simultaneously heating toner layers deposited on top of one another over a transfer material and fixing the toner layers. However, from the point of view of users who are familiar with printed materials, images produced by full-color copiers are still not satisfactory, and further improvement in image quality is demanded to achieve the high definition and high resolution of photographs and printed materials.
Conventionally, electric or magnetic latent images are visualized by toners. It is known that toners which are small in particle diameter and which have a narrow particle size distribution are used for improving the quality of electrophotographic images. Such toners are colored particles formed by adding a colorant, a charge control agent and other additives into binder resins, and methods for producing the toners are broadly divided into pulverization method and polymerization method. In the pulverization method, a toner is produced as follows: a colorant, a charge control agent, an anti-offset agent and the like are melted and mixed in a thermoplastic resin in such a manner as to be evenly dispersed, and the composition obtained is pulverized and classified.
The pulverization method is known to be capable of producing toners with fairly superior properties; however, selection of materials for the toners is limited. For example, compositions obtained through melting and mixing need to be able to be easily pulverized and classified by apparatuses. This demand requires compositions obtained through melting and mixing to be sufficiently brittle. For this reason, when the compositions are actually pulverized into particles, it is likely that a wide particle size distribution is created, and if an attempt is made to obtain copied images having excellent resolution and gray-scale properties, it is necessary to remove fine powders that are 5 μm or less in particle diameter and coarse particles that are 20 μm or greater in particle diameter through classification; hence, the pulverization method is disadvantageous in that the yield is very low. Also in the pulverization method, it is difficult for the colorant, the charge control agent and the like to be evenly dispersed in the thermoplastic resin. Uneven dispersion of additives has negative effects on the fluidity, image-developing properties and durability of the toners, image quality and the like.
In recent years, in order to solve these problems in the pulverization method, an invention concerning a suspension polymerization process has been disclosed, for example (Japanese Patent Application Laid-Open (JP-A) No. 09-43909). Also, there has been disclosed an invention concerning a process of effecting association amongst fine resin particles obtained by emulsion polymerization and thusly obtaining toner particles having indefinite shapes (Japanese Patent (JP-B) No. 2537503). Such polymerization toners are closer to spheres in particle shape than pulverized toners are, and ingredients are expected to be homogenized, so that they are superior in reproducing images. However, there is a new problem created in which the presence state of external additives such as a fluidity-adding agent typified by silica is liable to change on surfaces of particles.
It is inferred that the problem is due to the following. Since the surfaces of the toners are smooth and superior in packing properties, the toners have a larger number of contact points, and so the external additives will be easily embedded in the toners at the contact points between the toners or at the contact points between the toners and other members. As a method for solving such problems, there is a process of mixing fine resin particles having relatively large diameters or the like with particles.
Also, there has been disclosed an invention concerning a process of not only adding high-molecular-weight fine resin particles into toner particles but also localizing the fine resin particles on the surfaces of the toner particles to improve offset resistance (JP-A No. 2000-292978). However, this invention cannot sufficiently prevent external additives from being embedded because the minimum fixing temperature rises, the low-temperature fixing properties, in other words the energy-saving image-fixing properties, are not sufficient, and the high-molecular-weight resin is a component of the toner particles, so that there is no significant change in the shape of particles.